1. Field of the Invention
The present invention relates to a method of manufacturing a polysilicon thin-film transistor of a high electron mobility type.
2. Description of the Related Art
Research in active matrix liquid crystal displays (LCDs) has been developed. In an active matrix LCD, thin-film transistors are formed on a glass substrate. The thin-film transistor is used as a switching element for charging a pixel electrode to apply an electric field to a liquid crystal. Generally, the LCD is constituted by an amorphous silicon TFT (thin-film transistor), which is typically deposited on the glass substrate by a plasma CVD method.
In recent years, it has been considered to deposit on the glass substrate, polysilicon which has an electron mobility which is more than ten times greater than that of amorphous silicon. If the electron mobility of a thin-film transistor is increased, it is possible to construct a circuit for driving an LCD, a RAM, a ROM, and a CPU by using the thin-film transistor. This allows a large scale semiconductor device which cannot be formed by a semiconductor wafer.
There are two methods to deposit a polysilicon TFT on a glass substrate: one is to directly deposit polysilicon on a glass substrate; and the other is to deposit amorphous silicon on a glass substrate and thereafter anneal the amorphous silicon to form polysilicon. The former method is advantageous in efficiency, but disadvantageous in cost since the substrate must be formed of an expensive material which is resistant to a high temperature. In addition, the electron mobility cannot be satisfactorily increased. Although a low-temperature process has been developed, no remarkable effect has been published at present.
The present invention relates to the latter method. When amorphous silicon is deposited on a glass substrate by use of a conventional plasma CVD method, a substrate formed of an inexpensive material can be used, since the substrate is heated to a relatively low temperature. However, the method has the following drawback: amorphous silicon formed by use of the plasma CVD method includes hydrogen, which bursts away when amorphous silicon is polycrystallized into polysilicon by an annealing or heating process, resulting in a physical defect. To overcome this drawback, a method of depositing amorphous silicon by use of a low-pressure CVD method has been considered. Since amorphous silicon formed by the low-pressure CVD method includes little hydrogen, the burst of hydrogen in an annealing or heating process as in the plasma CVD method does not occur. However, since the substrate must be heated at a relatively high temperature, i.e., 500.degree. to 600.degree. C., it must be formed of an expensive glass substrate resistant to heat. Moreover, the conventional annealing or heating, which is achieved by CW (continuous wave) laser irradiation, produces polysilicon having inferior characteristics and a low electron mobility, i.e., about 10 cm.sup.2 /v.sec, since the polycrystallization by the CW laser irradiation is solid-phase growth.